1. Field of the Invention
The present invention relates to an aperture for a charged beam drawing machine and a method for forming the same, in a method for drawing a plurality of patterns on a semiconductor substrate in bundle by a charged beam such an electron beam or an ion beam.
2. Description of Related Art
Recently, with advanced micro-fabrication of a semiconductor device, lithography is now changing from a light exposure to a charged beam drawing (exposure), in particular, an electron beam drawing.
However, the charged beam drawing has a problem that although a high degree of resolution can be obtained, a throughput is low. In order to solve this problem, a method called a "cell projection" or a "block exposure" has been proposed.
As one example of this method, there is a method for causing a charged beam to pass through a transfer aperture (transfer mask) formed with a desired pattern, to shape the beam into a desired pattern shape, which is projected onto a semiconductor wafer.
This technology is disclosed by for example Y. Nakayama et al. "Highly accurate calibration method of electron-beam cell projection lithography", Proc. SPIE, Vol. 1924 (1993) pp 183-192.
A substrate material for the transfer aperture plate used in this method is a material which has an effect for shielding electrons and which can be easily shaped. The substrate material used here is a Si (silicon) substrate or an SOI (silicon on insulator) substrate,
Now, the transfer aperture plate in the prior art will be described with reference to FIGS. 1A to 1E.
First, as shown in FIG. 1A, on one principal surface (top surface) of a silicon substrate 31 having a thickness of 500 .mu.m to 650 .mu.m, a silicon oxide film 32 having a thickness of 1 .mu.m is formed, and a silicon layer 33 having a thickness of 20 .mu.m is formed on the silicon oxide film 32, so that a transfer mask substrate material 30 having the SOI structure as a whole is prepared. Here, the silicon oxide film 32 has an action as a bonding film for fixing the silicon layer 33 to the silicon substrate 31.
Then, a silicon oxide film 6 is deposited on a surface of the silicon layer 33, and after a resist layer is deposited, a resist pattern 7 is formed by means of a lithography.
Thereafter, as shown in FIG. 1B, the silicon oxide film 6 is patterned by a dry-etching using the resist pattern 7 as a mask, and then, the silicon layer 33 is patterned using the patterned silicon oxide film 6 as a mask. A pattern 33P of the silicon layer 33 thus patterned constitutes a transfer pattern of a transfer aperture.
In the prior art shown in FIGS. 1A to 1E, the thinned silicon oxide film 6 is removed after this step, but the silicon oxide film 6 may be left as it is.
Next, as shown in FIG. 1C, a silicon nitride film 8 is formed on the whole surface including a top surface, a bottom surface and a side surface. Thereafter, a silicon oxide film 9 having a thickness of about 0.1 .mu.m is deposited on the silicon nitride film 8 at the bottom surface, and a resist pattern 11 is formed on the silicon oxide film 9.
Furthermore, as shown in FIG. 1D, the silicon oxide film 9 is patterned using the resist pattern 11 as a mask, and then, the silicon nitride film 8 is patterned using the patterned silicon oxide film 9, so that a pattern 8P of the silicon nitride film 8 remains.
Then, the silicon substrate 31 is etched back from the bottom surface by a KOH solution using the silicon nitride film pattern 8P as a mask, and furthermore, an exposed intermediate silicon oxide film 32 is removed by the etching.
Thereafter, as shown in FIG. 1E, the silicon nitride film pattern 8P is removed with a heated phosphoric acid. Thus, there is obtained the transfer aperture plate having the silicon layer 33 having the transfer pattern 33P and a peripheral portion supported by a support member 35 formed of the silicon substrate 31 and the silicon oxide film 32.
Here, the reason for using the silicon nitride film 8 is that the silicon nitride film is an extremely excellent film since it has a high resistive property to the KOH solution and since a film deposition and a film removal are easy. In addition, since this aperture plate is used for manufacturing a semiconductor device, it is advantageous from the viewpoints of TAT and the cost that it is formed in a semiconductor device production line. Therefore, it is also convenient to use the silicon nitride film which is well used in a semiconductor device production process.
However, there is a problem in depositing the silicon nitride film 8. A method for depositing the silicon nitride film by using a conventional semiconductor process uses a LPCVD (low pressure chemical vapor deposition) process, which however requires a condition that a temperature is as high as 700.degree. C. to 800.degree. C. and a SiH.sub.2 Cl.sub.2 /NH.sub.3 gas is used at a flow rate ratio of about 1/10.
Since the film deposition rate is on the order of 0.7 to 3 nm/min., it is necessary to maintain the high temperature for a time of 35 to 140 minutes in order to obtain a film thickness on the order of 100 nm to 200 nm. Furthermore, adding a temperature rising time for elevating the temperature to the high temperature and a temperature dropping time for returning the temperature to a room temperature, the high temperature is maintained for 90 to 140 minutes.
The aperture plate for the charged beam drawing machine, as mentioned above with reference to FIGS. 1A to 1E, has a thin film region formed of the silicon layer 33, and a desired transfer pattern 33P is formed in this thin film region. In a process for forming the aperture plate for the charged beam drawing machine, when the silicon nitride film 8 is formed on the whole of the substrate (the whole of the wafer used as a material), since the film deposition is carried out in a high temperature condition, a warp occurs in the transfer aperture plate under influence of the intermediate silicon oxide film 32.
Namely, the silicon layer 33 having the film thickness of about 20 .mu.m exists between the uppermost surface and the intermediate silicon oxide film 32, but the bottom surface side silicon substrate 31 is as very thick as 500 .mu.m to 650 .mu.m in comparison with the silicon layer 33. Observing the whole (in the sectional view), the intermediate silicon oxide film 32 is biased to the top surface side, so that a stress occurs, which warps the aperture plate (wafer). After the silicon nitride film 8 is uniformly deposited on the warped aperture plate (wafer), the aperture plate (wafer) is returned to the room temperature. As a result, when the respective films are apt to return their original conditions, namely, when the warp is apt to disappear, a crack occurs in the silicon nitride film 8 at the bottom surface side because of a difference in thermal reduction between the respective films
If the crack occurs in the silicon nitride film, trash composed of silicon nitride is generated. In addition, if the crack has occurred in the silicon nitride film 8P shown in FIG. 1D, the bottom surface of the silicon substrate 31 as the support member becomes ragged with advancement of the etching using the KOH solution, and in this case, trash composed of silicon is also generated. In a patterning apparatus of not greater than 0.20 .mu.m, generation of trash in an area of a deposition apparatus must be severely suppressed. Furthermore, since the bottom surface of the silicon substrate 31 as the support member becomes ragged, a close contact with a holder is lost, which becomes a cause of a positional deviation.
Here, there is a case that no crack occurs in the silicon nitride film, depending the film thickness of the silicon nitride film. Even in this case, however, the warp has occurred in the finally finished aperture plate for the charged beam drawing machine, which becomes a cause for a pattern variation.